The invention relates generally to a device for feeding sheets to printing presses from a feed table to a continuously rotating impression cylinder of the printing press.
Devices of this kind are known generally. Pregripprs are known which, while rotating like stop cylinders or with a reciprocating swinging motion, grip every sheet registered against a feed table's front register lays and pull-type side register lay by means of grippers, accelerate the sheet to peripherical speed of the cylinder, and transfer it to grippers in the impression cylinder. Although such devices are operating with satisfaction, they are expensive and tend to bend the sheet according to the type of their design, this bending being detrimental particularly in relation to cardboard printing.
Known feeding devices of a different type clamp the registered sheet between feed-rolls and convey it on a nearly rectilinear way or path at an accelerated speed against stops on the impression cylinder, whereupon the cylinder grippers close. Such drive rolls have also been replaced with suction tapes with forward and backward motion. Such feeding devices push the leading forward of the sheet (push conveyors), whereas the devices mentioned first pull the sheets (pull conveyors). Advantages of the push conveyors are that they are less expensive and that they transfer the sheets to the cylinder grippers nearly free of curvature. It is a disadvantage however, of the push conveyors that the sheet driving means, -- either clamping rolls or suction tapes -- lie in front of the front register lays; i.e. between the front register lays and side register lays contrary to the first mentioned group of sheet feeding devices in which the pull conveyor is between the feed table front lays and the impression cylinder. The clamping or suction conveying device must therefore have finished sheet conveyance of a first sheet before the next sheet comes into its range of action. The path to the front register lays remains blocked up to that instant; as far as drive rolls are concerned, sheet conveyance must not be initiated before the trailing edge of the preceding sheet has cleared the clamping rolls. The period of time available for registering between one sheet and the next one is shorter than that available with rotating pre-grippers. Therefore, the conveying capacity of registered sheets is even lower with such push conveyors than with pull conveyors.